Semiconductor elements such as transistors, IC, LSI, etc., are usually resin-encapsulated by plastic packaging. Such a resin encapsulation is performed by casting, compression molding, injection molding, transfer molding, etc., in particular by transfer molding excellent in mass producibility and workability, using a thermosetting resin composition such as an epoxy resin composition, etc.
However, when the above-described transfer molding is continuously performed in the case of resin-encapsulating semiconductor elements with an epoxy resin composition by the transfer molding, a problem occurs such that the mold or die for molding is contaminated by a mold releasing agent in the epoxy resin composition, whereby good molding is not performed. That is, in the above molding, a mold releasing agent contained in an epoxy resin composition oozes out from the resin composition onto the inside surface of a cavity formed by the upper mold and the lower mold to cause mold-releasing action, but when the molding is repeated, the mold releasing agent oozed onto the inside surface of the mold is gradually accumulated thereon and deteriorated by being oxidized to form a hard oxidized mold-releasing agent layer (the surface of the layer being not smooth as the inside surface of the mold). Also, the molding is performed on the surface of the oxidized and deteriorated mold-releasing agent layer, whereby the surface pattern of the oxidized and deteriorated mold-releasing agent layer is transferred onto the surface of the molded article to roughen the surface of the molded article and give no gloss, etc. Also, such an oxidized and deteriorated mold-releasing agent layer is once formed on the inside surface of mold, in molding, thereafter, an epoxy resin composition, the mold-releasing agent further oozes out from the resin composition not to the surface of the mold but to the oxidized and deteriorated mold-releasing agent layer, whereby the mold-releasing agent cannot give sufficient mold-releasing action
For solving these problems, it has hitherto been practiced to introduce a thermosetting melamine resin moldable material into a mold when the oxidized and deteriorated mold-releasing agent layer is formed on the inside surface of the mold, thereby molding and curing the moldable composition, to unify the oxidized and deteriorated mold-releasing agent layer on the surface of the mold and the molded resin article in a unitary body, and then remove the molded article unified with the oxidized and deteriorated mold-releasing agent layer from the mold, whereby the surface of the mold is cleaned.
In this case, however, formalin is by-produced as a condensation product of the thermosetting melamine resin moldable material to emit an offensive odor, which results in reducing working environment and the cleaning workability.
Also, U.S. Pat. No. 4,670,329 discloses a cleaning sheet formed by coating a base with a thermosetting melamine resin moldable material. However, the use of such a cleaning sheet is undesirable since an offensive odor, etc., is emitted by the same reason as above at use.
Furthermore, in U.S. Pat. No. 3,476,599, there is proposed a method of introducing an uncured rubber series compound into the mold having the oxidized and deteriorated mold-releasing agent layer in place of the thermosetting melamine resin moldable material, curing the compound in the mold to form cured rubber, whereby the oxidized and deteriorated mold-releasing agent layer on the surface of the mold is unified with the cured rubber in a unitary body, and removing the cured rubber thus unified with the oxidized and deteriorated mold-releasing agent layer from the mold to clean the surface of the mold.
In this case, for improving the cleaning property, an aminoalcohol shown by the following general formula is added to the compound; EQU NH.sub.2 --ALK--OH
wherein ALK represents a straight chain or branched alkylene group having from 2 to 5 carbon atoms.
However, since the aminoalcohol is evaporated to emit an odor at curing the uncured rubber series compound, the above-described method also reduces the working environment and cleaning workability.
Also, when semiconductor element encapsulated with a resin by such a transfer molding, a part of the above-described moldable material attaches as burr to the peripheral portion of a cavity formed by an upper mold and a lower mold of the mold or the mold-releasing agent flows out and attaches to the peripheral portion to contaminate the mold.
Thus, it has generally been practiced to remove the contaminants such as burr, etc., attached to the peripheral portion of the cavity of the mold by rubbing the portion with a spatula or a brush and blowing away them by air blowing.
However, it takes a long time to clean the periphery of the molding cavity of mold using a spatula or a brush and also in such a case, a problem occurs such that the contaminants thus removed are scattered in the cavity of the mold to reduce the molding condition.
Also, transfer molding is generally carried out by automatic molding and hence in the above-described case, it is necessary to remove burr, etc., after once stopping the automatic molding operation, which results in reducing the production efficiency of moldings.